Integrated Weed Management

Integrated weed management involves the use of all available strategies to manage weed populations in a manner that is economically and environmentally sound. Such strategies include cultural, mechanical, chemical, and biological methods. Prune orchards may be infested with a variety of annual and perennial weeds. These weeds compete with prune trees for water and nutrients. Competition for these resources is of greater concern with young trees because weeds can reduce their growth, vigor, and productivity.

Weeds also cause problems in older orchards because they can increase the risk of frost damage early in the season, harbor pests and pathogens, interfere with irrigation systems, compete with the trees for water and nutrients, and interfere with harvest. There are several components to a good orchard weed management program. These include preventive strategies, orchard floor management, and weed monitoring. It is equally important to design a weed management program based on the irrigation system and soil type of the orchard. Further, proper use of pre- and postemergent herbicides and timely disking and cultivation are important factors in weed management.

Integrated weed management strategies vary from orchard to orchard. Location in the state, climatic conditions, soil texture and profile, irrigation practices, topography, and grower preferences influence orchard floor management decisions and the tools used. Weeds are commonly controlled either chemically or mechanically in a 4- to 6-foot-wide strip in the tree row. Resident vegetation is generally permitted to grow in the areas between the tree rows but it must be managed through mowing, tillage, or chemical treatment. Mulches, subsurface irrigation, flamers, and feeding by geese or other animals can also be used to control weeds in orchards. Often several weed management options are used in an orchard depending on the types of weeds present, age of the trees, soil conditions, and grower preference.

PREVENTION

A good weed management strategy in prune orchards begins with prevention, which is the most effective method of weed management. If possible, keep irrigation canals, ditch banks, and the irrigation systems free of weeds and weed seeds. A good drainage system is also essential as a preventive tactic. Fix leakages in the irrigation system and do not allow accumulation of water in low spots because moist sites encourage weed emergence and growth. Don't ignore weeds on the orchard margins because they produce seeds that disperse into the orchards. Therefore, control these weeds before they set seeds. Also, clean the undercarriage and tires of vehicles and equipment before entering the orchard because seeds and reproductive parts of weeds can be transported along with them.

MONITORING

Detection of new weeds and weed escapes is essential for preventing weed establishment or shifts in weed populations. Regular monitoring is a very important component of an integrated weed management program. For weed monitoring to be effective, the weed species present in and around the orchard must be properly identified. Identify weeds in the seedling stage because it is easier to control annual weeds with chemical or mechanical tools when they are small and have not become established. Perennial weeds are more vulnerable to control at the early bud stage or during fall when the plants begin to go dormant. Herbicides applied at these stages can be translocated to the roots or rhizomes to better kill the weed.

Most herbicides are only effective against certain weed species. Regular monitoring will determine if your treatments are working. Weeds often grow in patches so it may not be necessary to spray postemergent herbicides or cultivate the whole orchard to manage them. A spot treatment may save time and money while achieving good weed control. A handheld GPS unit is useful for marking patches of troublesome weeds for spot treatment and subsequent monitoring.

How to monitor

Survey your orchard for weeds in late fall and again in late spring. Keep records on a survey form that includes a map. Pay particular attention to perennial weeds and other problem weeds and note their location on the map. Record weeds found in rows and middles separately. Weeds in tree rows must be managed, but annual weeds in row middles may be beneficial as a cover crop. Also keep records of weed management actions including timing, rates and dates of herbicide applications and cultivations. Survey information collected over a period of years tells you how weed populations may be changing and how effective your management operations have been over the long term.

Late fall weed survey

Survey your orchard after the first rains of the fall when winter annuals have germinated. Monitoring weeds in fall accomplishes several tasks. It will identify any summer species and perennial weeds that escaped the previous year's weed control program. Adjustments can be made to control these species in the next year. Fall monitoring will also identify any winter species that are emerging. Record your observations on the fall weed survey form (PDF) and use the map to show areas of problem weeds.

Late spring weed survey

Survey your orchard in late spring or early summer, after summer annuals have germinated. By surveying weeds at this time, you can identify any species that escape control from earlier management and know what perennials are present. If herbicides were used, monitoring identifies any need for changing to another herbicide. Pay particular attention to perennials and check for their regrowth a few weeks after cultivation. Record your observations on the late spring weed survey form (PDF) and use the map to show areas of problem weeds.

ORCHARD FLOOR MANAGEMENT

A well-managed orchard floor cover between the tree rows has several benefits. Properly managed resident vegetation on the orchard floor provides a stable surface upon which machinery can be operated under wet conditions that otherwise would prevent access to the orchard. The plants in the orchard develop root channels that improve soil structure and water infiltration. Improved infiltration rates also reduce the risk of off-site movement of pesticides. Further, plant cover reduces soil compaction and the potential for erosion.

Resident vegetation

Although resident orchard-floor vegetation has several benefits, be sure that the vegetation does not invade the tree rows or it can result in a major problem, especially if the plants are difficult to control with herbicides. An example is hairy fleabane, which is difficult to control with the preemergent herbicides registered for prunes and is susceptible to postemergent sprays only when small. Its prolific production of wind-borne seed allows it to quickly invade tree rows.

Seeded cover crop

Planting a cover crop between the tree rows is an alternative to just letting resident vegetation grow. Choose a cover crop mix with known properties such as mowing height and frequency, time to seed set, and time to senescence. (For information on choosing a cover crop, see Covercrops for California Agriculture, UC ANR Publication 21471). Properly managed cover crops can prevent invasion of the orchard by weeds that cause problems. Tall cover crops or weeds, however, increase the risk of frost damage in spring and should be mowed or disked to reduce this risk.

IRRIGATION SYSTEM CONSIDERATIONS

Design weed management programs so that they fit the irrigation system. For example, the dissipation of preemergent herbicides is slow in furrow and basin flood systems with berms because the irrigation water does not come in contact with the herbicide. However, in sprinkler, microsprinkler, and drip-irrigated orchards the irrigation water contacts the herbicide, thus increasing its dissipation. Consequently, weed control provided by the preemergent herbicides breaks down sooner around the sprinklers or emitters compared to the rest of the orchard. Areas around sprinklers and emitters require additional weed control measures, such as a postemergent herbicide or hand weeding. For these treatments, using a sensor-controlled sprayer that applies herbicides only to areas of weed growth, similar to a spot treatment, can reduce postemergent herbicide use by 50% or more.

SOIL TYPE CONSIDERATIONS

Consider the soil type in an orchard when selecting a weed management strategy. Sandy loams to loamy sands require less herbicide for effective weed control than clay loams. Labels for preemergent herbicides have specific application rates for different soil textures. Applying the rate of herbicide suggested for a clay loam soil to loamy sand not only wastes herbicide but may also cause crop injury. Timing of cultivation is more flexible on loams and loamy sands than on soils high in clay because equipment moves easily through soil.

WEED MANAGEMENT BEFORE PLANTING

It is easier and cheaper to control perennial weeds before planting the orchard than after because there is a better selection of treatment options available when the ground is fallow. Established weeds can be controlled either chemically or mechanically. If the weeds are annuals, control them before they set seed by mowing, disking, or using herbicides. Perennial weeds can be mechanically controlled by repeated diskings in summer or controlled with herbicides.

A good time to control weeds such as dallisgrass, bermudagrass, and johnsongrass is the summer before planting. Apply glyphosate when the grasses are actively growing and then cultivate 2 weeks after the herbicide is applied. Many underground plant structures can be controlled by cultivation alone, which brings them to the surface and causes them to desiccate, but the soil must be dry for root systems of the perennial plants to completely desiccate and die. Many other weeds, including nutsedges, can be effectively controlled by cultivating with a soil-inverting plow.

Grading the orchard

Grade a new orchard site to ensure even drainage and to eliminate low spots that tend to promote perennial weed growth. Also, proper drainage prevents formation of wet areas within the tree row. Constant wetting accelerates the dissipation of herbicides, which leads to weed growth.

Preparing tree rows

Although a preemergent herbicide such as trifluralin can be incorporated in the future tree row before planting, treated soil must not be placed around the roots at planting or tree injury may result. When planting the trees, place untreated soil directly around the roots and then cover them with a surface layer of treated soil. During the early years maintain a weed-free strip that is at least 30 inches from trunk on each side of the tree to prevent weeds from competing with the developing tree. If planting holes are dug with an auger, use glyphosate before planting and then follow planting with an application of preemergent herbicide.

WEED MANAGEMENT IN NEWLY PLANTED ORCHARDS

In orchards that have received an herbicide treatment, disturb the soil as little as possible once the trees are planted. In orchards that are furrow irrigated, establish one or two narrow furrows along the planted trees. Perennial grasses can be controlled with clethodim (Prism), fluazifop-p-butyl (Fusilade), or sethoxydim (Poast). Glyphosate can be used to suppress nutsedges and perennial broadleaf weeds. Avoid spraying prune foliage or trunks with glyphosate. Wrappers may help to protect trunks from herbicides but there is no guarantee that injury will not occur. Regular preemergent and postemergent treatments during the establishment years remove much of the competition from weeds and facilitate irrigation and other cultural practices.

If herbicides were not applied before the trees were planted, weeds will need to be controlled. Cross-disking (cultivation both within and across the tree row) is an alternative to herbicide use but be careful not to injure tree roots when disking near trees or suckering can result and cause long-term problems if herbicides will be used in the future to control weeds in the tree row. This is especially important for trees planted on Marianna 2624 rootstock.

WEED MANAGEMENT IN ESTABLISHED ORCHARDS

If vegetation (either resident vegetation or cover crop) has been maintained in the orchard middles, it can either be mechanically managed by mowing or chemically managed by applying low rates of a postemergent herbicide that stunt the plants. An alternative to mowing is to let the cover crop grow until it is nearly mature and then roll it with a ring-roller to press the vegetation down. This accelerates the senescence process but allows some seeds to mature. On the other hand, the intact mulch blocks light that may prevent weed seeds from germinating. In early spring mow cover crops or resident vegetation to reduce the risk of spring frost damage.

Within the tree row, preemergent and postemergent herbicides are common management tools. For best results, most preemergent herbicides need to be sprayed onto the soil just before an irrigation or rainfall so that the water carries the chemical into the soil where the weed seeds are located. Check the pesticide label for specific details. Preemergent herbicides can provide control for up to a year, depending on the solubility of the material, adsorption of the material to soil, the weed species present, the dosage applied, and the amount of rainfall or irrigation that occurs. Herbicide leaching is greater on sandy than on clay soils. Prolonged moist conditions during winter, in furrow bottoms, or around low-volume emitters during irrigation favor breakdown and leaching of herbicides.

Postemergent herbicides are used on established weeds. They act either by contact or by translocation throughout the plant. Contact herbicides, such as paraquat, kill those parts of the plant that are actually sprayed, making good coverage and wetting essential. A single spray kills susceptible annual weeds. Re-treatment is necessary if perennials that regrow from underground roots or other underground structures are present or if annual weeds reestablish. Translocated herbicides, such as glyphosate, move into the plant and are translocated to the underground portions of the plant and kill them. Glyphosate, however, does not translocate into mature nutsedge tubers. Complete coverage with translocated herbicides is not essential, but does improve control. Complete control of established perennials is often difficult, because root structures are often extensive compared to the top growth.